A method and an internal combustion engine of this kind are disclosed in German Patent 195 06 980. There, the air/fuel mixture, which is supplied to the combustion chamber, is controlled in such a manner that alternately a rich air/fuel mixture (oxygen deficiency) and a lean air/fuel (oxygen excess) is present. The time intervals of the oxygen deficiency or of the oxygen excess are fixed in advance. The exhaust gasses generated during combustion are supplied to a catalytic converter which is provided, inter alia, for reducing the nitrogen oxides.
On the one hand, a catalytic converter of this kind operates as an oxidation catalytic converter. This means that, when there is a deficiency of oxygen, the oxygen is withdrawn from the nitrogen oxides and the hydrocarbons generated by the combustion and the carbon monoxides likewise so generated are all oxidized with this oxygen. For an oxygen excess, the oxidation catalytic converter could likewise reduce the nitrogen oxides. However, this reaction does not take place because of the oxygen present in excess and the oxidation catalytic converter uses the excess oxygen in lieu thereof.
On the other hand, the above-mentioned catalytic converter operates as a storage catalytic converter. This means that the nitrogen oxides, which are generated during combustion, are taken up by the storage catalytic converter when there is an oxygen excess. The storage catalytic converter releases the nitrogen oxides taken up when there is an oxygen deficiency.
By using the oxidation catalytic converter and the storage catalytic converter in the above-mentioned catalytic converter, the condition is achieved that the nitrogen oxides, which cannot be used by the oxidation catalytic converter when there is an oxygen excess, are taken up by the storage catalytic converter and are intermediately stored. When there is an oxygen deficiency, the nitrogen oxides, which are released by the storage catalytic converter, are reduced by the oxidation catalytic converter.
The storage catalytic converter can, however, only take up a limited mass of nitrogen oxides. This has the consequence that the storage catalytic converter must again be discharged after a certain loading time in which it takes up the nitrogen oxides. During the discharging, the storage catalytic converter again releases the nitrogen oxides so that it can be charged anew thereafter. If the storage catalytic converter is discharged too late, this has the consequence that the nitrogen oxides no longer can be taken up by this converter because of the "filled" converter and, therefore, escape as toxic substances into the environment. If the storage catalytic converter is discharged too long, it is then "empty" and no longer supplies nitrogen oxides so that the oxygen catalytic converter does not have the oxygen for oxidizing the hydrocarbons and the carbon monoxides whereby they escape to the environment as toxic substances.
The charging and discharging of the storage catalytic converter must therefore be controlled (open-loop control and/or closed-loop control). This is achieved by means of the oxygen inflow. During oxygen excess, the storage catalytic converter is charged and takes up the nitrogen oxides and, during an oxygen deficiency, the storage catalytic converter is discharged and releases nitrogen oxides. In the above-mentioned German Patent 195 06 980, the oxygen excess and the oxygen deficiency are controlled over initially fixed time intervals. However, this has been shown to be too imprecise.